Generally, cellular radio communications networks comprise several cells each including a base station and mobile terminals. A communication between a base station and a mobile terminal is carried out by means of two communication channels: a communication channel in the uplink direction so that the terminal communicates to the base station and a communication channel in the downlink direction so that the base station communicates with the terminal. In a cell, a communication channel in the uplink direction and a communication channel in the downlink direction for a same communication are generally symmetrical. In fact, these communication channels have a same pass band width, a same modulation and thus have consequently a same data rate. In addition, the communication channels are matched initially, in other words for a communication channel in the uplink direction corresponds a communication channel in the downlink direction, the carrier frequencies comprising respectively said channels being separated by a duplex gap generally fixed in the frequency band dedicated to the network.
Cellular radio communications networks are subdivided into public networks and private networks using different technologies.
A large part of voice communications in public networks are simultaneous communications from one subscriber to another subscriber. Such a communication may be divided into two sub-communications, a sub-communication corresponding to a communication in a same cell between a terminal of a subscriber and a base station.
In a different manner, voice communications in private networks are mainly group communications in which each member of the group via his mobile terminal speaks in turn in addressing the other members of the group. The different members of the group may be spread out over a geographic area including several cells, in general adjacent. In each cell, is allocated for a communication, a pair of communication channels comprising an uplink communication channel and a downlink communication channel that are matched and symmetrical. A member of the group who speaks, uses his mobile terminal to communicate his message to the base station of the cell in which it is located, by means of an uplink communication channel. The base station of the cell and the other base stations of the cells covering the communication group retransmit the message to the other mobile terminals of the group by means respectively of downlink communication channels. In a group communication covering several cells, the uplink communication channels are statistically less used than the downlink communication channels, by a factor approximately equal to the number of cells covering the group communication, implying as a result an asymmetric communication traffic. The asymmetry of the communication traffic of private communication networks using pairs of matched and symmetrical uplink and downlink communication channels do not make it possible to optimise the spectral efficiency of the network. In fact, in certain communication technologies, such as the technology linked to the TETRA (“TErrestrial Trunked Radio”) standard, for a transmission of a message a single uplink communication channel is used, the other uplink communication channels not used allocated to the same group communication are reserved because matched to the downlink channels also allocated to the same group communication, preventing the use of said uplink channels by other group communications.